Field of Invention
The present invention relates to a flyback power converter, a secondary side control circuit, and a control method thereof; particularly, it relates to such flyback power converter which adaptively adjusts a frequency of a zero of a compensator gain function and/or a mid-frequency gain of the compensator gain function according to a load current, and a secondary side control circuit and a control method thereof.
Description of Related Art
In applications of a power converter for providing power to a motor or a light emitting device (LED) circuit, a two-stage power converter is often required in order to achieve power factor correction (PFC) and to provide an adjustable output voltage (e.g., switchable between different levels such as 5V, 12V, and 24V etc.). For example, as shown in FIG. 1A, an AC power supply 30 generates an AC voltage Vac. The power converter 100 includes a first stage PFC circuit 10 and a second stage DC-DC conversion circuit 20. The first stage PFC circuit 10 converts the AC voltage Vac to a DC output voltage Vout. The second stage DC-DC conversion circuit 20 converts the output voltage Vout to a load current Iload which is provided to the load circuit 40, wherein the load circuit 40 is for example a motor or a light emitting device circuit. The first stage PFC circuit 10 is often embodied by an isolated circuit, in order to isolate the output voltage Vout and the AC voltage Vac. The second stage DC-DC conversion circuit 20 may be embodied by a switching regulator or a linear regulator, and if desired, a dimmer function of the light emitting device circuit may be provided in the second stage DC-DC conversion circuit 20.
As one example, the power converter 100 shown in FIG. 1A can be an isolated LED driver circuit with a dimmer function, wherein the first stage PFC circuit 10 operates in a discontinuous conduction mode (DCM) to achieve high PFC and to isolate the LEDs from the AC power. The second stage DC-DC conversion circuit 20 regulates the load current Iload by a linear regulator, for adjusting the brightness of the LEDs.
In the aforementioned power converter 100, typically, the control system of the power converter 100 is compensated by fixed parameters which are designed for heavy load condition. In this case, when the load current Iload is low, i.e., when the load circuit 40 is in light load condition, an unstable situation of a control system may occur because the compensation is improper. FIG. 1B is a schematic diagram showing signal waveforms of the output voltage Vout and the load current Iload in the unstable situation. As shown in FIG. 1B, when the load circuit 40 is in light load condition, the output voltage Vout and the load current Iload will be conductive intermittently, which will cause flickers of the LEDs that is undesirable.
In view of the above, the present invention proposes a flyback power converter, a secondary side control circuit, and a control method thereof, which solve the light load unstable problem. For example, when the present invention is applied to driving LEDs, the brightness of the LEDs can be maintained stable without flickering even in the light load condition.